When patients and their doctors choose treatments for cancer, they must consider their beliefs about likely treatment benefits and side effects. Patient co-morbidity and disability are no doubt factored into this decision, but clinical evidence for these decisions seldom exists. Decisions are made with great uncertainty as to the optimal treatment. This is clearly the case in early stage prostate cancer (ESPC) where it is not known whether "aggressive treatment," i.e., radiation therapy or radical prostatectomy, affords significantly better outcomes than conservative treatment, i.e., "watchful waiting". We previously used instrumental variables methods to analyze the SEER-Medicare database and found evidence that, among men diagnosed with ESPC during 1986-1993, aggressive treatment was under-utilized for men aged 66-70 as increases in rate of aggressive treatment would have increased 5-year survival rates. We also found significant evidence that aggressive treatments were over-utilized for men aged 81+, as decreases in the aggressive treatment rate would not have lowered 5-year survival rates. However, since 1993, treatment rates have continued to expand, and aggressive treatment methods have been refined. In addition, newer and more refined versions of the SEER-Medicare databases are now available. The purpose of this pilot project is to determine whether aggressive treatment of ESPC continued to be underutilized among younger men (age 66-70) and over-used among older men (age 81+) diagnosed during 1994-1999. Specific aims are to: 1) describe the factors (e.g. age, co-morbidity, tumor grade, access to healthcare) that are related to the sorting of patients into conservative or aggressive treatments; 2) estimate treatment effectiveness (three- and five-year survival) for patients on the extensive margin using instrumental variables techniques. Estimate for: (1) conservative vs. aggressive treatment and (2) given aggressive treatment, radiation vs. prostatectomy; and 3) isolate the set of patients for whom our estimates can be generalized to help policy makers and clinicians to create effective interventions to change practice behavior. Collectively, these analyses of men diagnosed from 1986 through 1999 will provide robust evidence on which to justify primary data collection including medical record reviews and patient and physician surveys to determine the role of patient preferences, patient and physician perceived efficacy and attitudes, patient resources, and clinical characteristics in ESPC treatment decision-making. These details will be critical for guiding interventions to expand or contract the rate of aggressive treatments among subgroups of the population.